1. Field of the Invention
The present invention relates to a method of allocating channel codes for synchronous uplinks in a mobile communication system using a plurality of scrambling codes for the uplinks.
2. Description of the Related Art
In CDMA mobile communication system, there are downlink and uplink between mobile stations and a base station where a downlink is a data path from a base station to mobile stations and an uplink is for reverse direction.
In a cell of a base station, a plurality of channels of a downlink are synchronized between the base station and many mobile stations, so that interferences between channels arise little in decoding with mutually orthogonal codes for each channel.
However, channels of an uplink are asynchronous each other since mobile stations are located in different sites in a cell, so that orthogonality between channels can not be used. This inherent problem causes interferences between uplink channels. Furthermore, such interferences are increased in proportion to increment of uplink channels, namely, active mobile stations, therefore, the capacity of an uplink is inevitably limited.
To increase the capacity of an uplink, it is needed to synchronize uplink channels by adjusting transmitting time of each mobile station. If uplink channels are synchronized each other, it is possible to minimize interferences between uplink channels with mutually orthogonal codes. A related technology is USTS (Uplink Synchronous Transmission Scheme).
The USTS technology is for increasing capacity of an uplink by guaranteeing orthogonality between uplink channels. In CDMA (Code Division Multiple Access) mobile communication system, as aforementioned, it has no benefit to use mutually orthogonal codes for an uplink from mobile stations to a base station because channels of the uplink are not synchronized, whereas it has great benefit to use mutually orthogonal codes for a downlink because all channels of a downlink are synchronized. Therefore, the capacity of CDMA system is totally dependent on the uplink capacity which is restricted by interferences between uplink channels proportional to the number of active mobile stations.
The USTS technology, which aims at synchronizing uplink channels to resolve such problems of CDMA mobile system, distinguishes mobile stations, namely uplink channels with mutually orthogonal channel codes while conventional method distinguishes mobile stations with different scrambling codes. Since channel codes are used for differentiation of channels, channel codes of one scrambling code may be insufficient, therefore several scrambling codes may be used in a cell of a base station to produce more channel codes. If multiple scrambling codes are used, it is necessary to develop a new method for allocating channel codes which can maximize CDMA system capacity.